This invention relates to a process for oxidizing H.sub.2 S, and particularly to a process for catalytically oxidizing H.sub.2 S to sulfur, SO.sub.2, or both in the presence of a substantial proportion of water vapor.
Current air pollution regulations are very restrictive concerning the amount of H.sub.2 S that may be discharged to the atmosphere. In some instances, gas streams may not be discharged to the atmosphere if they contain more than about 10 ppmv of H.sub.2 S. Thus, many processes have been developed to remove H.sub.2 S from gas streams prior to their discharge to the atmosphere.
One method known in the art for removing H.sub.2 S involves catalytic oxidation, that is, a gas stream containing H.sub.2 S is blended with air or free oxygen, and the resulting mixture is then passed through a bed of catalyst particles under appropriate conditions such that the H.sub.2 S is converted to elemental sulfur vapor or SO.sub.2, or both, as desired. One catalyst useful for the gas phase conversion of H.sub.2 S to sulfur or SO.sub.2 is disclosed in U.S. Pat. No. 4,092,404; it comprises one or more vanadium oxides or sulfides supported on a refractory oxide such as alumina or silica-alumina. Another such catalyst is disclosed in U.S. Pat. No. 4,012,486, wherein a catalyst having active components consisting of bismuth is used to catalytically incinerate H.sub.2 S to SO.sub.2.
When compared, the bismuth catalyst of U.S. Pat. No. 4,012,486 will generally be found to be less active than the vanadia catalyst of U.S. Pat. No. 4,092,404 for oxidizing H.sub.2 S. On the other hand, a bismuth catalyst is much more stable than a vanadia catalyst when H.sub.2 S must be removed from a gas stream, such as an off-gas derived from a geothermal power plant, which contains water vapor at a water vapor partial pressure of 4.0 psia or more. In general, at water partial pressures below about 1.0 psia, vanadia catalysts have satisfactory stability while at water vapor pressures above about 1.0 psia, and particularly at 1.5 psia or above, vanadia catalysts deactivate rapidly. It is believed that the reason for this deactivation is due to a complex series of chemical reactions involving the conversion of the vanadium oxide or sulfide active catalytic components to less active forms of vanadium, such as vanadyl sulfate (VOSO.sub.4).
Accordingly, it is an object of the invention to provide a stable catalyst having high activity for the oxidation of H.sub.2 S in the presence of water vapor. It is another object of the invention to provide a process for catalytically oxidizing H.sub.2 S to SO.sub.2, elemental sulfur, or some percentage combination of both, as desired. It is another object of the invention to provide a process for oxidizing H.sub.2 S in the presence of water vapor at a partial pressure of more than about 1.0 psia. It is another object of the invention to provide a process for selectively oxidizing H.sub.2 S in the presence of such components as H.sub.2, CO, NH.sub.3, and CH.sub.4. Other objects and advantages will be apparent from the following description of the invention.